PARENT GRANT ABSTRACT Aging has a dramatic effect on regeneration of all tissues, bone included. When a young adult fractures a bone, stem cells residing within the bone marrow cavity proliferate and differentiate into osteoblasts. This process results in regeneration of the skeletal element with complete restoration of its biomechanical properties. In the elderly, however, this process is often inadequate, which will result in a delayed healing response and subsequent medical complications. Chronic inflammation mediated by activation of the NF-?B pathway has recently been linked to a decline in regenerative potential in aged skin, skeletal muscle and nervous system, although the direct effects of NF-?B activation on stem cell function in these tissues is still under investigation. Accumulation of senescent cells in aging tissues has been suggested as the driver of chronic inflammation, as these cells secrete inflammatory factors, which affect the entire surrounding cellular milieu, further stimulating inflammation. The mechanisms underlying this age-related decline in stem cell function are the focus of this proposal. We hypothesize that the accumulated effects of chronic inflammation lead to osteoprogenitor cell dysfunction. Within this proposal, we will quantify and qualify age-dependent alterations in progenitor cell frequency, proliferative and osteogenic differentiation potential, and determine if these changes are caused by chronological aging or age-associated inflammation. While previous studies have demonstrated a reduced regenerative potential of heterogeneous bone grafts from aged animals, this study will for the first time assess the affect of aging on a homogeneous progenitor cell population in an in vivo environment. In the second part of this proposal, we will investigate the underlying mechanism of action that is responsible for increased osteoprogenitor cell senescence and resulting regenerative dysfunction. Here, we will identify the machinery that activates telomere dysfunction and cell senescence. From this experiment, we will learn, whether chronic inflammation is the culprit for reduced regenerative function of the aged bone progenitor cell, and whether this process is reversible. Finally, we will study the effect of chronic inflammation on the regenerative decline of the aging osteoprogenitor cell in a regenerative context and will utilize the information gained in the previous aims to design a therapeutic approach that will lead to the rejuvenation of skeletal progenitor cells. The goal of this highly translational project is to identify the basis of the age-related decline in osteogenic progenitor cell function and thus to define a new therapeutic target for improved bone healing in the elderly patient.